Titanium oxide finds use in a variety of applications, for example, pigments, UV screeners, catalysts, photocatalysts, catalyst carriers, adsorbents, ion exchangers, fillers, reinforcements, ceramic stock materials, precursors to complex oxides such as perovskite type complex oxides, and primers for magnetic tape.
Among others, photocatalytic titanium oxide fine powder is a material having photocatalytic activity upon exposure to light including UV light of 400 nm or shorter. The photocatalytic activity refers to the action that holes and electrons generate upon excitation by UV light of 400 nm or shorter, diffuse to a surface, and act on molecules adsorbed to that surface to undergo redox reactions. By the redox reactions, organic matter is decomposed when adsorbed to the titanium oxide surface.
Once titanium oxide fine particles having such photocatalytic activity are applied to the substrate surface to form a photocatalyst thin film, harmful organic materials adsorbed thereto can be decomposed by irradiating excitation light. They are often used in applications including cleaning of the substrate surface, deodorizing, and sterilization. It is required to increase the contact area of photocatalyst particles with a material to be decomposed for the purpose of enhancing photocatalytic activity, and film transparency is required for the purpose of preserving the aesthetic appearance of a substrate to be coated. To meet these requirements, titanium oxide fine particles in the titanium oxide dispersion must have an average particle size of not more than 50 nm.
Furthermore, titanium oxide exerts satisfactory photocatalytic activity upon exposure to light in the UV region of relatively short wavelength (wavelength 10 to 400 nm) in sunlight or the like, but is sometimes awkward to exert satisfactory photocatalytic activity in an indoor space with lighting from a light source mainly producing light in the visible region (wavelength 400 to 800 nm) such as fluorescent lamps. Recently, a tungsten oxide photocatalyst (Patent Document 1: JP-A 2009-148700) attracts attention as the visible-light-responsive photocatalyst. Since tungsten is a rare element, it is desired to enhance the visible light activity of a photocatalyst using a common element, titanium.
The general processes for preparing titanium oxide fine particles, which are industrially implemented, include the sulfate process using ilmenite ore or rutile ore and the chloride process (Non-Patent Document 1: Titanium Oxide, Gihodo Publishing Co.), as well as the hydrolysis/calcination process, reaction in organic solvents, and the solid phase process (Non-Patent Document 2: Standard Research of Photocatalysts, Tokyo Tosho Co.). The resulting titanium oxide fine particles are subjected to ultrafine dispersion treatment in a coating liquid in order that titanium oxide fine particles be coated onto the substrate surface while maintaining the aesthetic appearance of the substrate to be coated. The general fine dispersion treatments include, for example, methods of dispersing titanium oxide fine powder as synthesized in dispersing media with the aid of dispersing agents such as organic dispersants by a wet dispersing machine (Patent Document 2: JP-A H01-003020, Patent Document 3: JP-A H06-279725, Patent Document 4: JP-A H07-246119, Patent Document 5: JP-A 2004-182558), and methods of surface-treating titanium oxide so that it may be dispersed in dispersing media in a stable manner (Patent Document 6: JP-A 2005-170687, Patent Document 7: JP-A 2009-179497). However, these manufacturing methods have the problem that since ultrafine particles with an average particle size of not more than 50 nm are prone to agglomerate, a great deal of labor is necessary in order that even primary particles be dispersed, and in some cases, it is impossible to disperse even primary particles; and the problem that when particles are surface-treated with inorganic or organic components or when dispersing agents such as surfactants are added, both for the purpose of enhancing dispersion stability, the photocatalyst is surface coated therewith, which becomes a factor of inhibiting the photocatalyst from exerting activity.
Also disclosed are the method of preparing an anatase type titanium oxide dispersion having long-term stability by hydrothermal treatment of a peroxotitanic acid solution obtained by dissolving titanium hydroxide with the aid of hydrogen peroxide (Patent Document 8: JP-A H10-67516); the method of preparing rutile type titanium oxide sol (Patent Document 9: JP-A H02-255532); and the method of preparing titanium oxide sol (Patent Document 10: JP-A H10-182152). In these dispersions, titanium oxide fine particles are kept dispersed without a need for surface treatment or dispersing agents while the average particle size is not more than 50 nm. Photocatalytic coating films obtained by coating substrates with the dispersions exhibit high transparency and activity upon UV light exposure, but not sufficient visible light activity.